This invention relates to electrical pulsing systems. More particularly, the invention relates to pulsed electrical power circuits for high repetition rate gas lasers. Specifically, the invention relates to circuits in which high power, high voltage, fast rise time, narrow electrical pulses provide electrical energy for exciting a gas mixture, thereby producing laser operation.
Electronic transition lasers, such as rare gas excimer, dimer, and charge transfer lasers, offer scalable high energy photon sources in the ultraviolet and visible wavelengths. These lasers can be scaled to high pulsed output energies by increasing the volume, pressure, and energy deposition into a high pressure rare gas halide mixture contained within the laser cavity.
Rare gas halide electronic transition lasers operate on several fundamental principles. First, an intense source of electron excitation is used to excite a mixture of rare gas plus a small percentage of a molecular additive. A selecte rare gas is a dominant component of the mixture because the electron excitation is initially deposited into ionization and excitation of the rare gas. Second, high total pressures are necessary for effective operation of these lasers. Large volume, self-sustained glow discharges, however, are extremely difficult to achieve at high pressures unless special precautions are taken because high pressure glow discharges tend to rapidly form an instability and convert to an arc discharge. Arc discharges constrict into small streamers, thereby eliminating the volume excitation necessary for proper laser operation. Accordingly, an efficient high pressure electric discharge for rare gas halide electronic transition lasers is characterized by a discharge that is volumetric and stable (i.e., does not degenerate into a constricted arc). Third, the electron energy in the discharge should be high enough to produce sufficient rare gas ions and metastables. There should also be a sufficiently high current density in order to produce a sufficient number of excited rare gas species in a short time period which is less than the time required to react all of the molecular additive. These three criteria require a high power, high voltage discharge circuit incorporating some method for stabilizing the discharge to prevent arcing.
Many problems have been encountered in the implementation of discharge circuits necessary to produce the high electron temperatures required in high pressure rare gas buffered mixtures. Typical problems are that the gas tends to break down at too low a voltage. Also, discharges at electron energies necessary for efficient pumping of the laser tend to constrict into an arc unless the discharge pulse has a very steep rate of rise (under 100 nanoseconds) and the duration is kept shorter than the arc formation time or the time to react all the molecular additive. The gas after breakdown has a very low impedance (i.e., less than several ohms) which necessitates a low impedance discharge circuit for efficient energy deposition into the laser load. High voltage charging circuits typically have too large an inductance to provide either a rapid voltage rise time or a sufficiently low output impedance for optimum energy transfer to the laser load. Consequently, a key problem associated with these lasers is the development of an efficient, long-lived, nondestructive, nonablative reliable and inexpensive method of electron energy deposition into the laser load. The pulse rise time shaping, pulse width compression, and impedance matching electrical excitation circuit of the present invention provides an effective solution to the problem.
U.S. Pat. No. 4,275,317 discloses a circuit for the purpose of efficient energy transfer from a relatively slow high power, high voltage charging circuit to a laser load. The circuit comprises one or more saturable inductor switches, each of which has an associated distributed capacitance energy storage device. Energy is provided to a distributed capacitance energy storage device by a voltage source and is contained therein by a saturable inductor switch. When the energy build-up reaches a predetermined level, the saturable inductor switch becomes saturated, thereby allowing the energy to flow therethrough and into either a next intermediate capacitance energy storage device or the laser load.
The basic operation of the circuit disclosed in U.S. Pat. No. 4,275,317 is that of a Melville line. W. S. Melville, Proceedings of The Institute of Electrical Engineers, Vol. 98, Part III, Number 53, pp. 185-207, May, 1951. With the aid of FIG. 1, a capacitor C.sub.1 is charged rather slowly by an external charging circuit with a saturable inductor L.sub.1 saturating at the peak voltage on the capacitor C.sub.1. When the saturable inductor L.sub.1 saturates and switches to a low inductance, a capacitor C.sub.2 is charged more rapidly to near the same voltage. A saturable inductor L.sub.2 saturates at the peak voltage on the capacitor C.sub.2, charging a small capacitor C.sub.3 in an even shorter time and providing a discharge current for the laser load through an inductor L.sub.D.
Since the unsaturated inductance is not infinite, some prepulse voltage appears across the laser load of the circuit disclosed in U.S. Pat. No. 4,275,317. The purpose of the capacitor C.sub.3 is to reduce the magnitude of prepulse to prevent laser breakdown. The prepulse amplitude and voltage rise time are both inversely proportional to the value of the capacitor C.sub.3. Thus, a tradeoff between prepulse and rise time occurs. Since the current does not reverse between charging and discharging, a magnetic diode across the laser as used in the present invention would not be practical with the circuit disclosed in U.S. Pat. No. 4,275,317. The electrical excitation circuit in accordance with the present invention reduces the amplitude of the prepulse across the laser load without an increase in rise time.
When the laser discharge begins, a large fraction of the discharge current flows from the capacitor C.sub.2 through the saturable inductor L.sub.2 of the circuit disclosed in U.S. Pat. No. 4,275,317. The effective laser discharge inductance is therefore L.sub.D +L.sub.2 (saturated). The total inductance in the discharge loop must be minimized for optimum laser performance. The inductance of the saturable inductor L.sub.2 (saturated) will be somewhat above the air core value and can be as low as a few nanohenries with careful construction. The discharge loop inductance (neglecting the inductance of the saturable inductor L.sub.2) is typically a few nanohenries, and, therefore, the saturable inductor L.sub.2 represents a significant increase in total loop inductance. In accordance with the present invention, an electrical excitation circuit is provided which avoids any added inductance in the discharge loop.
U.S. Pat. No. 4,275,317 discloses that distributed capacitance energy storage devices must be utilized in order to provide the high voltage, narrow pulses required by electronic transition lasers. Distributed capacitance energy storage devices which can be used include coaxial lines, multiple coaxial lines, parallel plate transmission lines, or two or more parallel-connected capacitors having an associated natural or added inductance for creating a pulse shaping network.
U.S. Pat. No. 4,275,317 further discloses that in order to achieve efficient operation of the laser, a pulse shaping network providing less than 10-nanosecond rise time pulses with durations in the hundred nanoseconds region must be used, and, therefore, the saturable inductor switch must have characteristics, and be constructed, in a manner differing from that of conventional saturable inductor switches. That is, the saturable inductor switch must be formed of a material having a very high permeability and a cross-sectional thickness on the order of the skin depth of the material at a frequency corresponding to the desired rise time of the pulse. For many types of lasers a high voltage 10-nanosecond rise time pulse is desirable, and, therefore, the skin depth criterion requires that the material thickness be on the order of one to two microns. Magnetic material films of this thickness can be obtained by deposition on a plastic insulator backing. These backings can be formed into a tape which is then wound around a suitable nonmagnetic core material, thereby creating the saturable inductor switch.
The circuit disclosed in U.S. Pat. No. 4,275,317 requires distributed capacitance energy storage devices and saturable inductor switches which must be tailored for a specific laser configuration, and are both difficult to fabricate, and expensive. In accordance with the present invention, an electrical excitation circuit is provided which is adjustable for different laser configurations, is largely fabricated with readily available commercial components, and is relatively inexpensive.